The present invention relates to a switching SW regulator control circuit of the PWM type which can supply a large current to a load even with a low-input power supply voltage in accordance with an input power supply voltage of a switching regulator (hereinafter referred to as "SW regulator") and which can prevent from flowing a larger current than required one even in an over-load state such as a load short-circuiting, at the time of a high-input power supply voltage.
As a conventional PFM SW regulator control circuit, there has been known a SW regulator control circuit shown in a circuit diagram of FIG. 5. That is, there is provided an error amplifier 13 that amplifies the differential voltage between the reference voltage Vref of a reference voltage circuit 10 and a voltage of at a node of bleeder resistors 11 and 12 that divide an output voltage Vout of an output terminal 5 of the SW regulator. Assuming that the output voltage of the error amplifier 13 is Verr, the output voltage of the reference voltage circuit 10 is Vref, and the voltage of the node of the bleeder resistors 11 and 12 is Va, if Vref&gt;Va, Verr becomes high, but conversely if Vref&lt;Va, it becomes low.
A first circuit, which is a PWM (pulse width modulation) comparator 15 compares an output Vtri of an oscillator circuit 14, for example, a triangular wave with an output Verr of the error amplifier 13 to output a signal. This is shown in FIG. 6. That is, the pulse width of the output Vpwm of the PWM comparator 15 is controlled by making the output voltage Verr of the error amplifier 13 larger or smaller. During this pulse width period, the switch element used in the SW regulator is on/off-controlled. This is a so-called PWM operation of the SW regulator.
Also, an output Vcomp of a second circuit, which is a comparator 17 that compares a reference voltage value Vref2 of a reference voltage circuit 16 with a triangular wave output voltage value Vtri of the oscillator circuit 14 becomes low in level when Vref2&lt;Vtri. When the output Vcomp of the comparator 17 becomes low in level, its low level output is inputted to an AND gate 18, and an output Vand of the AND gate 18 is always low in level. That is, the maximum duty ratio of the SW regulator (the maximum value of the ratio of a period during which the switching element used in the SW regulator is on to the switching period of the SW regulator; hereinafter referred to as "maximum duty ratio") is determined by setting the reference voltage value Vref2 at a certain level of the triangular wave Vos of the oscillator circuit 14. In general, in case of the SW regulator, if a period during which the switching element used in the SW regulator is on is longer, the capacity for supplying an electric power to a load increases. For example, as the load becomes heavy, that is, as the output load current value becomes large, the output voltage of the SW regulator drops, and the voltage Va divided by the bleeder resistors drops. As a result, since the output voltage Verr of the error amplifier 13 increases with the result that the pulse width of the output Vpwm of the PWM comparator 15 is widened (the duty ratio becomes large), the pulse width is controlled so that the output voltage Vout is held constant.
Conversely, as the load becomes light, that is, as the output load current value becomes small, the output voltage of the SW regulator increases, and the voltage Va divided by the bleeder resistors increases. As a result, since the output voltage Verr of the error amplifier 13 decreases with the result that the pulse width of the output Vpwm of the PWM comparator 15 is narrowed (the duty ratio becomes small), the pulse width is controlled so that the output voltage Vout is held constant.
That is, the output voltage Verr of the error amplifier 13 varies in accordance with the load current value to control the pulse width of the SW regulator.
In general, in case of the SW regulator, an energy stored in a coil used therein depends on the voltage difference between both ends of the coil, that is, an input power supply voltage, and the higher input power supply voltage, the higher the energy stored in the coil. That is, even in a same load, the lower input power supply voltage requires a larger pulse width to turn on the switch element used in the SW regulator, and a higher input power supply voltage turns on the switch element used in the SW regulator by a smaller pulse width.
However, the conventional SW regulators suffer from the following problems. That is, if the maximum duty ratio is made small, when the power supply voltage is low, the coil energy due to the switching operation is small with the result that a large load current cannot be supplied. Also, if the maximum duty ratio is made large, when the power supply voltage is high, even in an abnormal state such as when load short-circuiting occurs, because the coil energy due to switching operation is large, a large current flows in the power supply circuit or the SW element to cause damage thereto.
In view of the above, in order to solve the above problems, an object of the present invention is to provide power supply voltage dependency to the maximum duty ratio of the SW regulator of the PFM system so as to set the maximum duty ratio to be large when the power supply voltage is low, and to set the maximum duty ratio to be small when the power supply voltage is high, so that energy can be sufficiently supplied to a load even at a low power supply voltage, and a power supply current and a switching current (a current flowing in the switch element used in the SW regulator) are suppressed, even when abnormality such as load short-circuiting occurs, when the power supply voltage is high.